This invention relates to a fuel injection valve for use in a direct-injection type internal combustion engine.
In a conventional diesel engine, there is a problem that while the fuel injection rate, i.e. injection quantity per unit time should be set at a suitably high value for improvement of the output characteristics of the engine as well as prevention of the emission of nitrous oxides, an increased fuel injection rate can lead to a correspondingly shortened injection period, which causes a reduction in the combustion duration, resulting in a combustion noise, and can also cause an increase in maximum pressure within the engine cylinders.
On the other hand, in conventional fuel injection valves using multihole nozzles for conventional diesel engines, the valve operation is monotonously affected by pressure changes within the injection pipe connecting between the valve and the fuel injection pump, that is, a specific amount of increase in the pressure within the injection pipe causes the same amount of increase in the lift of the nozzle needle in a lower injection quantity region as in a higher injection quantity region. As a consequence, in the lower injection quantity region, the nozzle needle can lift through an excessive stroke to provide an excessive injection quantity due to the increase of the pressure within the injection pipe, and the resulting pressure drop in the injection pipe causes too small an injection quantity during the next injection stroke, followed by an excessive injection quantity during the further next injection stroke. In this manner, the conventional fuel injection valves suffer unstable or irregular fuel injection. Although conventional fuel injection valves using pintle nozzles can overcome the above phenomenon of unstable or irregular injection by virtue of the arrangement that the nozzle needle has its tip slidably fitted in the injection hole in the nozzle body to produce a throttling effect, those using multihole nozzles are unable to avoid the above phenomenon.
To solve the above problem, a fuel injection valve has been proposed by the assignee of the present application in U.S. Ser. No. 237,941 filed Feb. 25, 1981 for instance, which includes a second nozzle spring arranged in the nozzle holder in addition to a conventionally employed first nozzle spring. In this proposed fuel injection valve, fuel injection takes place in two steps, i.e. an initial injection and a main injection, in such a manner that during the initial injection the nozzle needle is lifted through a limited stroke against the force of the first nozzle spring, and the main injection subsequently takes place with the valve opening pressure determined by the combined force of the first and second nozzle springs. This double-step injection reduces the injection rate throughout the whole injection period, thus substantially overcoming all the aforementioned drawbacks of combustion noise, emission of nitrous oxides and irregular injection.
According to the above proposed fuel injection valve, during the initial injection lifting of the nozzle needle causes corresponding lifting of a first movable spring seat which supports the first nozzle spring, to cause contraction of the same spring to execute an initial injection lift, and during the subsequent main injection the first movable spring seat is further lifted to urgingly displace a rod-like second movable spring seat which supports the second nozzle spring, to cause contraction of the same spring to execute a main injection lift. Further in the above fuel injection valve, the valve opening pressure for initial injection is determined by the setting load of the first nozzle spring, and the valve opening pressure for main injection by the sum of the setting loads of the first nozzle spring and the second nozzle spring, respectively.
However, according to this injection valve, adjustment of the setting load of the first nozzle spring is carried out by selecting the thickness of a shim interposed between the same spring and its seating surface formed in the nozzle holder, which requires dismantling the injection nozzle portion of the valve for each adjustment of the initial injection valve opening pressure and the main injection valve opening pressure. This makes the adjusting operation complicated. In addition, during the dismantling operation, dust can be intruded and caught in gaps between the nozzle holder and the nozzle body, causing leakage of fuel, clogging of the injection holes, etc. Further, to adjust the initial injection lift, an adjusting threaded member, which is threadedly fitted in the nozzle holder, is rotated to axially displace the second movable spring seat engaging therewith to vary the gap between the first movable spring seat and the second movable spring seat. However, like ordinary threaded parts, machining tolerances exist between the screw threads formed on the outer peripheral surface of the above adjusting threaded member, the associated inner peripheral surface of the nozzle holder and the inner peripheral surface of a lock nut threadedly fitted on the adjusting threaded member, which can result in play of the adjusting threaded member relative to the nozzle holder. Also, due to the machining tolerances, the line which connects between a crest of each screw thread and a root diametrically corresponding to the crest and crosses with the axis of the screw thread is not normal to the axis of the screw thread. As a consequence, when the adjusting threaded member is fastened by the lock nut, it can get dislocated from its adjusted position. On the other hand, the initial injection lift value of a fuel injection valve of this kind is very small, i.e. on the order of 0.1 mm. Therefore, it is very difficult to set the initial injection lift gap to a desired value with accuracy.
Furthermore, fuel within the injection valve can leak through the lock nut along its upper and lower surfaces. Further tightening of the lock nut for prevention of the fuel leak can cause a further dislocation of the adjusting threaded member from its adjusted position due to the above machining tolerances, distortion of the screw thread, etc.